Some birds have the ability to sleep while they are in flight, such as frigate birds. Other birds, like warblers, take long breaks at rest stops during their migration.
Let’s meet the frigate bird. This large black bird is slightly bigger than a crow and male frigates have a beautiful red throat that puffs up when they are trying to attract a female mate. These seabirds can be found on various coasts around the world, from Florida to the Indian Ocean, and they spend most of their time searching for food and hanging out on the coasts where they live.
Meet the (male) frigate bird. (Photo Credit : Uwe Bergwitz/Shutterstock)
However, some frigate groups migrate to warmer shores after the breeding season. This journey can last for several months and involves continuous flying, day and night, with only a few breaks in between.
If they fly day and night, when do they sleep? And it’s not just frigate birds that do this; many birds undertake challenging long-distance flights. Since sleep is essential for most living organisms, they must find a way to sleep. So, the question is… how and when do they sleep?
How Birds Sleep While Flying?
A research paper published in Nature Communications in 2016 studied frigate birds during a 10-day flight to understand their sleeping patterns. The researchers used electroencephalography (EEG) to record the brain’s electrical activity while the frigate birds were flying long distances. What they discovered made headlines – frigate birds can sleep while flying!
Anyone who has tried to sleep while standing knows that it’s impossible to control any muscle, let alone keep one’s arms “braced for flight,” right? However, birds have a different sleeping pattern than mammals like us.
Frigatebirds lounging. The Frigate birds that have a red throat are the males, while the white and black birds are the females. (Photo Credit : Gerry Bishop/Shutterstock)
REM Sleep
The researchers found that frigate birds take short and well-timed naps. These naps come in two types, with the first being REM sleep, which lasts for 10 seconds. REM sleep is the stage when we dream. In mammals, REM sleep can last for several minutes, but for birds, especially those that are flying, it only lasts a few seconds. Nonetheless, REM sleep is crucial for proper brain function.
Uni-hemispheric Slow Wave Sleep
The other type of sleep that birds experience while in the air is called uni-hemispheric slow-wave sleep. Slow-wave sleep is the deepest phase of Non-Rapid Eye Movement (NREM) sleep.
The term “uni-hemispheric” means that birds put only half of their brain to sleep while keeping the other hemisphere awake. In fact, birds keep the eye connected to the awake hemisphere open. They do this to remain vigilant of predators or any other potential threats, and many birds even do this when they are not migrating.
Although frigate birds sleep during their flight, they prefer to stay awake most of the time, spending less than 3% of their time sleeping. This suggests that frigates can regulate the amount of sleep they need. Once they reach land, they make up for their lost sleep and spend almost half of their time dozing.
In 2006, scientists who were studying the Swainson’s thrush, known as Catharus ustulatus, discovered that the bird takes multiple 9-second power naps during migration. Unlike the previous research on frigate birds, this study was conducted on captive birds.
Sleeping In Pit Stops
Currently, frigate birds are the only birds that have been studied and shown the ability to sleep while flying. It is possible that many other birds use this strategy as well, but they have not been studied yet.
However, since sleeping during flight is not very restful, there must be another way for birds to catch up on sleep. Many migratory birds take occasional breaks to rest and refuel before continuing their journey.
For example, the garden warbler, known as Sylvia borin, breeds in Europe and Asia during the summer and then migrates to sub-Saharan Africa before the winter begins. To successfully complete this journey of about 4,000 km (or 2,500 miles), the birds make pit stops to eat and sleep. Researchers from the University of Vienna, the Max Planck Institute, the Swedish University of Agricultural Sciences, and the University of Rhode Island collaborated to study how these birds sleep during their Mediterranean pit stops.
The researchers observed that the bird’s sleeping behavior varied depending on how tired they were from their flight. The more tired birds slept in shrubby vegetation, their preferred roosting spot, with their head tucked between their wings. This allowed them to conserve energy and lose less heat. In contrast, the less tired birds slept without tucking their heads away, remaining more alert and aware of their surroundings. Although this meant they used more energy during sleep, it also allowed them to respond faster to predators compared to the birds that tucked their heads away.
Ultimately, individual birds make a trade-off between sacrificing complete sleep to quickly escape predators or getting a good night’s sleep and risking being eaten.
Some Birds Don’t Sleep
There are some birds that minimize their sleep or even avoid it altogether. Research on the Alpine Swift, known as Tachymarptis melba, found that the bird can fly continuously for almost 200 days. However, there is still limited evidence of the bird sleeping, although there have been periods of decreased activity that resemble sleep-like behavior.
Other birds, such as the male pectoral sandpiper, known as Calidris melanotos, sacrifice sleep during mating season due to intense competition. Research on these sandpipers has confirmed this behavior.
The male sandpipers that had the least amount of sleep fathered the highest number of offspring. Their lack of sleep did not affect their performance or competitive edge in the mating process. This finding, although not related to migration, suggests that birds are capable of adjusting their sleep patterns without experiencing significant physiological consequences. The challenges faced by Alpine Swifts are not to be underestimated.
Diversity in Bird Sleep Patterns
The behaviors of birds vary greatly when it comes to sleep. While some birds, like frigate birds, are able to sleep while flying, others, like warblers, take rest breaks. On the other hand, certain birds, such as sandpipers, have completely abandoned sleep. This wide range of sleep patterns among birds makes them an intriguing subject for researchers, particularly those studying migratory birds. However, it can also be challenging to study them due to their individual differences in sleep behavior and response to sleep deprivation.
Fortunately, advancements in technology, such as the use of EEG in studying frigate birds and the ICARUS project, which utilizes the satellite on the International Space Station (ISS) to track migratory birds, offer promising opportunities for researchers to gain insights into the remarkable journeys of migratory birds and their ability to function without sleep, a phenomenon that seems to defy conventional understanding.